This study investigates ionic current rectification (ICR) in funnel-shaped nanochannels coated with polyelectrolyte (PEL) layers through detailed continuum simulations. Three different PEL distributions, including uniform, cylinder only, and bipolar, are examined over a wide salt concentration range. Unlike prior studies that focused solely on geometric or surface-charge asymmetry, our results reveal a synergistic enhancement when combining funnel geometry with a bipolar PEL coating. The coupling of the funnel geometry with opposite surface charges induces asymmetric depletion–enrichment coupling zones that amplify ionic selectivity and rectification beyond additive contributions. This cooperative mechanism achieves a rectification factor up to 10 times higher than that of single-asymmetry designs, demonstrating a new design paradigm for nanofluidic diodes. A parametric sweep reveals an optimal fixed charge density, beyond which electrostatic saturation reduces the rectification despite further surface charging. Geometric studies show that shortening the cylindrical section and widening the tip diameter each boost the rectification factor. Yet, high rectification is accompanied by a markedly reduced absolute current in the reverse bias direction, underscoring a fundamental trade-off between selectivity and throughput. These insights into the coupled roles of charge distribution, channel geometry, and PEL density provide design guidelines for high-performance nanofluidic diodes in ionic circuits, sensors, and blue energy harvesters.
